SEOUL (AFP) - Tens of thousands of activists protested in Seoul on Sunday against the Group of 20 Summit to be held this week, with hundreds clashing with police who used pepper spray to disperse an angry crowd. Labour campaigners and other activists - numbered by police at 20,000 and by organisers at 40,000 - chanted slogans and songs at Seoul Plaza outside the city hall, surrounded by thousands of riot police. We will never allow only 20 elite countries to decide the whole worlds future, leaders chanted from the stage, as they also protested over the state of workers rights in South Korea and a proposed free trade deal with the US. Hundreds of protesters tried to march towards the city centre against police warnings, pushing and shoving against police riot shields. Police used pepper spray to disperse the demonstrators. Thousands of others staged a candlelight rally at Seoul Plaza, surrounded by buildings draped with huge banners heralding the November 11-12 summit - the nations biggest appearance on the world stage since the 1988 Olympics. Some activists wore vests with slogans reading: Against the G20 that hampers labour rights and creates unstable jobs and Against the G20 that cuts social welfare and destroys public service. A South Korean, meanwhile, has been arrested for sending a terror threat against this weeks G20 summit to embassies of Muslim nations, police said Sunday. The man, age 46 and identified only by his surname Kim, was arrested Friday after faxing the threat to the Seoul embassies on Thursday, police said. The message read: Hello, Allahs warriors? An event Osama bin Laden likes has been prepared at the G20 summit. We wish that your country will be full of Allahs blessings. The fax in Korean was sent to the embassies of Saudi Arabia, Indonesia, Pakistan, Afghanistan, the United Arab Emirates, Kuwait, Tunisia and Turkey, police said. The arrested man did not appear to present a genuine threat, a spokesman for Seoul city police told AFP. He just seems to be a person who harbours a lot of disaffection about the government, he said. He will be charged with disruption of business. It looks like he tried to cause a stir among Muslim countries and unnerve the public ahead of the G20 Summit, the Yonhap news agency quoted a police official as saying. Kim had owned a medical equipment business but it collapsed in the wake of the 1997-98 Asian financial crisis, leaving him in financial difficulties, according to Yonhap. The agency quoted Kim as saying he was frustrated that world leaders are having a feast under the name of G20 summit when ordinary folks are suffering from the economic downturn caused by advanced countries faulty financial policies.